This invention relates generally to electromagnetic flowmeters, and in particular to a meter of this type which measures the flow rate of a slurry containing ferromagnetic particles of a like magnetic substance, the meter providing an accurate measurement regardless of the ferromagnetic concentration.
An electromagnetic flowmeter is adapted to measure the flow rates of those fluids which present difficult handling problems, such as corrosive acids, sewage slurries, detergents and the like. In a flowmeter of this type, the liquid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by a pair of diametrically-opposed electromagnets. When the flowing liquid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator, or to supply an input variable to a process control system.
The electromagnetic force signal established across the electrodes of the flowmeter is a function of the flow velocity of the fluid being metered. This electromagnetic force depends on the specific permeability of the fluid which, in the case of water, is approximately equal to unity. But where the fluid being metered takes the form of a slurry containing ferromagnetic particles or like magnetic substances, the specific permeability of the fluid becomes significantly larger than unity, thereby increasing the electromagnetic force signal.
When, therefore, an electromagnetic flowmeter calibrated to measure the flow rate of water is used to meter a ferromagnetic slurry, the indicated flow rate will be in error to an extent that depends on the concentration of the ferromagnetic slurry. If the slurry concentration is substantially constant, then the flowmeter can be corrected to take this concentration into account. But if the concentration is subject to change in the course of operation, then, as a practical matter, one cannot make the necessary corrections.